Apparatus for producing multi-ply paperboard products



June 6, 1961 c. s. SWEITZER ET AL 2,987,122

APPARATUS FOR PRODUCING MULTI-PLY PAPERBOARD PRODUCTS Filed OCT. 12, 1955 INVENTORS. (Zr/ LE: 3 Smnrznz a/v .Beocx,

ATTO RN EYS.

United States Patent 2,987,122 APPARATUS FOR PRODUCING MULTI-PLY PAPERBOARD PRODUCTS Charles S. Sweitzer, Middletown, and Jon Brock, Butler County, Ohio, assignors, by mesne assignments, to Diamond National Corporation, a corporation of Dela- Ware Filed Oct. 12, 1955, Ser. No. 540,125 3 Claims. (Cl. 162-304) This invention relates toapparatus for the manufacture of paperboard such as may be used in the formation of printed cartons and like employment; and it has for a principal object novel apparatus for the manufacture of a paperboard of multi-ply construction with one or more plies having unique properties.

It is an object of the invention to provide apparatus to produce a paperboard having one or more outer liners which are exceedingly thin, but very uniform, fiat and smooth, having near normal density and better folding characteristics than conventional liners.

It is an object of the invention to provide apparatus producing a paperboard having one or more outer liners with superior printing and varnishing qualities and capable of receiving a mineral-adhesive coating to better eflfect.

The objects of the invention include the provision of apparatus utilized in processes for the manufacture of such liners and for the manufacture of paperboard including such liners.

Additional objects include the provision of apparatus for controlling the nature of the paperboard including the nature of the said outer liners, in such matters as water resistance or absorption, opacity, strength and color.

These and other objects of the invention which will be set forth hereinafter or will be apparent toone skilled in the art upon reading these specifications, are accomplished in those products and by those procedures of which certain exemplary embodiments will hereinafter be set forth. Reference is made to the accompanying drawing which is diagrammatic in nature, but which illustrates essential parts of the novel apparatus used to carry on the process, and in which:

FIGURE 1 is a diagrammatic representation of a portion of a board machine, and

FIGURE 2 is a diagrammatic cross-section of a suction drum mold assembly.

FIGURE 3 is a partial perspective view with elements broken away showing the outer construction of the suction drum.

Paperboard is ordinarily manufactured on multicylinder board machines. These machines comprise a series of conventional cylinder screen molds, each rotating about three-quarters submerged in its own vat of dilute paper pulppulp having a consistency of /2 by weight being illustrative. Water from the pulp passes through the screening on the surfaces of the cylinder molds so that each forms on its submerged surface a thin, endless layer of wet paper which emerges as the cylinder molds rotate.

The layer of wet paper is picked up by a moving, endless papermakers woolen felt, the felt being urged against the layer of wet paper on each cylinder mold successively by a rubber-surfaced couch roll pressing the felt against the cylinder mold. Additional layers are picked up from successive cylinder molds until the felt carries on its under surface an adhering endless web of wet paper of the desired number of plies, say, six or seven in a conventional operation.

There are several well-known varieties of cylinder molds differing from each other in respects such as the configuration of the vat circle within which the cylinder mold rotates, dimensions of the molds, the manner and direction of supplying the dilute paper pulp to the cylinder molds, and the like. All such varieties are usable in the process hereinafter outlined.

A top felt is led against the composite web, which is next subjected to the nip pressures of a series of press rolls, some with and some without suction, and later to the action of press rolls of great smoothness without a felt intervening on the side being smoothed, until the water content of the web has been reduced to some 65% by weight, and the web is adequately self sustaining. It next passes in serpentine fashion and free of the felts, over a series of heated dryer rolls to the extent of drying the board to a water content of some 3 to 5% by the time the end of the series is reached. Finishing operations may ensue, including calendering, coating and the like.

The normal paperboard of commerce is a multi-ply product. As distinguished from other paper making operations in which a web of the desired weight or thickness is formed in a single layer, the multi-cylinder mold operation has certain advantages. The nature of the pulp may vary as between the vats of the several cylinders, singly or in groups, affording the opportunity to make the body of the board of relatively inexpensive stock with top and in some instances back liners of more expensive materials. Top liner plies conventionally are made of higher quality fibrous stocks than other plies. Filler plies are often made to contribute extra strength to the board. The heating and refining treatments given the stocks used in the several vats may be varied to contribute to the workability of the stocks on the cylinder molds and to the properties of the finished plies of paper. The skilled worker in the art will recognize that there is a wide variety of types of pulp stocks and treatments of them which may be employed. The top liner of a multi-ply paperboard must meet standards not only of appearance but also of printing quality. A typical, though by no means limiting, paperboard may have a body (several interior plies) of chip or the like, chip comprising a mixture of waste papers of all kinds. It may have a back liner rich in news or the like for appearance sake, news being printed newspapers; and it will have a top liner of a stock appropriate to the use to which the board may be put. Thus the top liner may be rich in virgin chemical wood pulps, bleached or not bleached, such made appropriately by the sulphite, sulphate, soda, etc., processes, the remainder of the furnish being similar fibrous materials derived from graded waste papers. The liner may also have a content of groundwood fiber where lightfastness is of no moment. The liner probably will contain a finely divided mineral or pigment such as clay or titanium dioxide; it may contain a starch; and it will be made to a given standard of whiteness, brightness or color, and may be sized to a desired degree of water re sistance. It may be designed for direct printing or may be intended for coating with a mineral-adhesive mixture, all of which may dictate variations in its composition well known in the art.

Since the masking power and smoothness of a top liner in conventional practice may not be sufliciently adequate to cover and conceal the inequalities and appearance of the body plies, a ply of materials generally between top liner and body stock in quality and expense is used between the top liner ply and the upper body ply. This socalled under, inner or second liner may likewise vary widely as to composition, as is well known.

Multi-cylinder paperboard also possesses properties sometimes advantageous, sometimes otherwise. In the first place it is a highly directional product due to its manner of formation. The mode of supplying the fibrous stock to the cylinder molds and the rotation of the molds themselves tends strongly to orient the fibers in the direction of travel of the formed web; and the fibers in all of the plies making up the product tend to be similarly oriented. This gives to the resulting product highly directional characteristics, i.e. the tear and tensile strengths and the stiffness of the board are not the same with and across the direction of travel of the formed web on the board machine. So great are the differences that they must be taken into account in cutting and scoring for the formation of cartons of adequate stiffness and foldability.

Moreover the thickness and uniformity of the individual plies is subject to continuous sporadic and/or repetitive variations, both across and lengthwise of the board, being afiected by the manner in which the pulp stock moves and flows in the vats as it approaches the screen surface of the rotating cylinder mold, the uniformity and speed of rotation of the cylinder mold, the specific concentration of pulp stock in adjacent areas, the degree of mechanical perfection of the cylinder mold and its screen surface, etc. Thus portions of a cylinder mold ply will tend to contain a greater number of individual fibers than immediately adjacent portions, the fibers being also formed together in an irregular Way. For these reasons the masking or covering power of a cylinder mold liner tends to be inadequate, or putting this in another way, adequate masking or covering power in a cylinder mold liner necessitates a greater average thickness of liner than would be the case if the uniformity of the liner could be substantially improved.

The orientation of the fibers in a cylinder mold liner tends to be directionally the same as the orientation in the body plies, and this causes an accentuation of the difference in folding quality of the board between the acrossand with-machine directions of the web and, under certain circumstances, a splitting of the surface of the board at score lines, particularly those in the with the-machine direction of the web, impairing the appearance of cartons and the printing thereon.

These defects have led in the past to various suggestions for remedy including the idea of laminating a separately formed relatively thin Fourdrinier paper layer to conventional multi-ply paperboard. This, however, is normally too expensive.

The Fourdrinier paper machine, as is well known, comprises a long endless belt of wire screen supported by large diameter rolls at each end. At the starting end of the upper flight is the breast roll; at the other end where the wire reverses its travel is the couch roll. Be tween these the upper flight is additionally supported by a series of revolving small diameter table rolls followed by one or more suction boxes. In the slower running Fourdrinier machines, particularly, the wire section is provided with an oscillating device to impart a mechanical shake to all or a portion of the moving screen, thus to assist in dispersing the fibers of the paper pulp on the wire screen and to decrease their tendency to become oriented in the direction of travel of the screen. Dilute paper pulp, say, of 0.10 to 1.4% fiber is run on to the upper flight of the moving Wire at the breast roll through a slice (there are several varieties of slices) extending completely across the Wire. As the pulp issues onto the wire, water immediately starts draining from it therethrough assisted by the suction boxes, leaving the fibers formed into a web on the wire itself. At the couch roll the damp but formed web, containing usually some 18% to 22% oven dry fiber, is transferred to a felt and thence passes through a series of press rolls, smoothing presses, heated drying rolls, finishing calenders and the like, to emerge finally as finished paper.

A Fourdrinier 'web lacks the plied characteristics of multi-cylinder boards and also is much less strongly directional. 'At the same time, the Fourdrinier operation, while excellent for the manufacture of relatively thin paper products, particularly those of high quality, and for the manufacture of paper products in very large volume, has difiiculty competing with the multi-cylinder operations in the manufacture of paperboard or boxboard, particularly the thicker varieties which are difficult, expensive or impossible to form in one layer on the Fourdrinier machine. In the use of Fourdrinier web formation, it is substantially impossible to effect a saving by making the body of the product of a stock varying from and cheaper than the stock of the usual surface or surfaces, although it is known how to impose a second layer of stock on a first layer on a Fourdrinier wire.

Applicants carried on considerable experimentation in an endeavor to combine in a single machine a plurality of cylinder molds and a short Fourdrinier screen with a suction breast roll in the manufacture of a multi-ply product. Various difficulties were encountered which do not need to be outlined here.

It was noted in attempting to use a short Fourdrinier screen section that practically all of the felting of the fibers occurred over the breast roll. This led to the development of a dilferent type of felting device referred to hereinafter by the term suction drum mold, which is unlike both the cylinder screen mold and Fourdrinier wire. This is illustrated in the drawing wherein 1 is a hollow metal drum roll with a perforated surface. In an exemplary but nonlimiting embodiment the roll 1 has a diameter of about 40 inches and a length a little greater than the width of the paperboard web being made. The roll is mounted with shafts, journals and bearings and is powered individually to rotate with a peripheral speed equal to that of the paperboard web being assembled on the bottom felt of a board machine. The entire roll surface is perforated with staggered holes 16 of of an inch diameter, spaced of an inch between centers, drilled radially (FIGURE 3). Solidly set into the drum roll surface are thin metal fins 17, placed parallel and longitudinal of the drums length, about 1 of an inch apart and projecting about A of an inch above the roll surface. The fins, in turn, are notched to receive a wire 18 wound spirally around the roll with convolutions M; of an inch apart. Covering the wire and the entire roll surface are two layers of wire screening, 19 and 20, the inner one having 14 meshes to the inch, and the outer one meshes to the inch. This arrangement of holes, fins and winding wire permits the drainage of water passed through the screens with a minimum of wire-mark on the formed ply.

Inside and extending substantially the full length of the drum roll there is a suction box 2, adjustable as to its radial position but held in a fixed, non-rotating position in use. This suction box is divided into two sections by a solid wall 3 as shown. The angular spread of the upper section is fixed, as of about 35, while the lower wall of the lower section is movable to provide a maximum spread of 60 and minimum of 30, by way of example. Provision is madefor sliding sealing strips a, b and 0 between the ends of the suction box walls and the inside surface of the drum roll. The suction box is, of course, connected to a water and vacuum pump through one or more conduits (not shown) and extending through the shafts of the drum roll. The suction box being divided into sections, it is possible to vary the suction in each section.

A means is provided for delivering pulp to the suction drum mold. This is a hopper-like feed conduit or stock distributor 4 extending substantially the full length of the drum roll. The feed means is characterized by spaced lips, the lower lip 5 being held at a very short distance from'the wire screen surface ofthe drum roll, say, at 0.030 inch distance, and the upper lip 6'at about 3 inches. The position of the lower lip is preferably at or near the lower edge of the bottom section of the su'ction'box as shown, and the upper lip generally at or near the wall separating the two sections of the-suction" box. Owing to the upward rotation of the drum roll the leakage of watery pulp at the lower lip is negligible in amount but beneficial because it keeps the lip clean of fiber bunches, dirt, etc., which otherwise would collect there as when the p makes a sliding contact with the wire screen surface.

The side edges of the pulp-feeding conduit 4 are approached closely to the wire screen surface. Side plates 21, clamped against conduit 4 and holding sealing strips against the screen surface may be used for this purpose, and they extend a short distance beyond the upper lip of 4.

Dilute paper pulp with a dry fiber content of say up to 0.1 to 0.2% by weight is pumped continuously to and through the feeding means in such fashion that it approaches the screen surface of the suction drum mold substantially radially at all points across the length of the mold, and at a rate calculated to maintain the desired coverage of mold surface. This involves pumping the pulp in excess to the screen surface whereby an added advantage accrues in the elimination of dead spots in the flow of pulp to the screen surface and a consequent improvement of formation of the fibrous web on the surface. The excess spills away outwardly over the upper lip of the feeding device as shown by the arrow, and is collected for reuse. Without resort to the overflow the problem in maintaining a desired coverage of screen surface with pulp is acute but not insurmountable.

A relatively strong suction is maintained inside the suction box, say 5 inches of mercury. This suction rapidly filters the fibers out on the wire screen covering of the rotating suction drum mold, and the fibers have but little time to become oriented in the direction of motion of the screen surface. As the lengthening web moves with the rotating roll (clockwise in FIGURES 1 and 2) and out of the head of watery pulp, suction from the remaining areas of the suction box continues to remove water from the web and to consolidate it. It will be noted that in the direction of rotation of the roll, free watery pulp is brought into contact with it only over a relatively short span. The white water is collected for reuse in diluting and/or preparing the supplies of watery pulp.

Continuing on, on the surface of the suction drum mold and formed damp web moves into contact with the outer one of a series of previously formed and still wet plies of paper material 7 clinging to the underside of the moving bottom felt 8 of the machine. The felt is pressed against the mold by a pressure roll 9, and as the felt 8 leaves the suction drum mold it carries with it the layer of paper previously formed thereon.

The wire-screen-covered surface of the suction drum mold iskept clean by means of a series of high pressure water sprays issuing from a pipe extending across the length of the suction drum mold at a position closely following the removal of the wet paper from its surface. This position is shown at 15 in FIGURES 1 and 2.

With difierent amounts of suction in the two chambers of the suction box some control of the quality of the freshly formed web can be expected. For example, with increased suction on the web after it has emerged from the head .of watery pulp at the feeding device a more compact and drier (but still damp) ply results than with the same suction applied in both areas. On the other hand, with less suction in the second box section, reduced compactness and increased wetness results. In creased suction in the first suction box section also tends to reduce directional characteristics of the Web by affording less time for the stock to align itself in the machine direction- These factors play a part in the matter of ply adhesion, hereinafter discussed. The specification for the suction drumnrold and attendant devices given above are illiistrative'but not limiting. The skilled worker in the art 'will understand that they can be varied for different effects, or in. connection with different speeds and differentqualities of stock.

However, a new type of web or ply is formed when the suction drum mold device is used. The web is remarkably uniform and level, can be made very thin, and because of its remarkable uniformity and continuity a very thin ply can completely cover underneath plies. vMoreover, the directional characteristics of the web may be reduced from those of the thick web made by the conventional cylinder mold.

The suction drum mold and attendant apparatus is suitable for use with ordinary cylinder molds in forming a composite, multi-ply paperboard having new and valuable characteristics as set forth above. In FIGURE 1 there is illustrated a single cylinder mold assembly comprising the conventional vat 10 and cylinder mold 11, operating in the usual fashion. The ply of pulp formed on the cylinder mold is picked up by the bottom felt 8 acted upon by a pressure roll 12.

A multi-ply board machine could be made up of pulp molds all of which are suction drum molds, there being such a number of these as is required to produce a web of the desired thickness. It would likewise be possible to alternate cylinder molds and suction drum molds in a board machine. Since, however, the cylinder mold is particularly adapted for the making of fairly heavy or bulky plies useful to form the body of a multi-ply web, and since the thinner webs normally produced by suction drum molds have especially valuable characteristics as liners, it is preferred to provide in a board machine a series of cylinder mold assemblies to form the body layers, and one or two suction drum mold assemblies to form a top liner or both a top liner and a second liner. The back liner of ,a paperboard product can usually adequately be formed by a cylinder mold, especially since this liner is not normally intended for printing; but it is within the purview of the invention to provide a suction drum mold assembly to produce a back liner also. The greater the number of layers contained in the board which are produced by suction drum mold devices the more the complete board will have the properties of the suction drum mold type of ply. Where directional characteristics of the individual plies have been minimized, the multi-ply board will have them minimized too. By the use at least as a top ilner of a ply formed through the use of the suction drum mold assembly, however, a multi-ply paperboard can be formed with all of the valuable and novel characteristics set forth in the objects of this invention, as stated above, including good folding characteristics.

In joining together plies in the wet state, especially where one ply is formed on a suction drum mold assembly and an adjacent ply is formed on a cylinder mold, a problem of adhesion may be encountered. The probable mechanism of ply bonding will now be discussed briefly. The literature on this subject is not extensive; reference will be made principally to the doctoral dissertation of Duncan S. Brown of applicants assignee, entitled The Relation of the Strength Properties of Multiply Paperboard to the Bonding Between Plies, presented June 1939 at the Institute of Paper Chemistry, Appleton, Wisconsin, and published in The Paper Trade Journal, vol. 112, No. 3, pages 33-34, Jan. 16, 1941, and to a published article of W. B. Campbell of The Pulp and Paper Research Institute of Canada entitled Fibre Structure and Its Influence in Paper Making, published in The Paper Trade Journal, vol. 100, No. 7, pages 35-38, Feb. 14, 1935.

When plies of felted fibers are separately formed, pressed wet to a compact damp condition and then are laid up to provide a multi-ply web, the whole being pressed and dried, a number of factors appear to control the extent to which the plies will be bonded to each other. The interfaces or meeting surfaces of the plies do not appear to be felted together in the sense of having the surface fibers inter-twined, inter-twisted or the like, although a minor degree of'felting may occur. Rather,

the exposed fibrous surfaces at the interfaces appear to be' bound together by adhesive forces. For example it can be shown that when-two damp cellulose fibers of proper quality are pressed together in. sideby side relationship and, are dried, they will adhere-to each other. a

This is explained. by. Campbell. on the theory that all cellulosic surfaces in water. are covered with a layer of cellulose in partial solution, and that when such surfaces are brought into contact with each other they will tend to unite by the crystallizing action of the partially. dissolved cellulose when the water evaporates.

Brownin his thesis drew. certain conclusions of interest here based on considerable research in the bonding of plies in multialayered webs.-, The bonding of plies in a multisply web is generally-similar to the bonding of fibers inside the plies. The, strength of thebond between fibers in a-ply, aswell as between fibers in adjacentpliescan be increased by increasing the area of actual contact between the fibers. This probably. explains why. the strength of a cylinder mold Web is so highlydirectional. The fibers tendto line up in the machine direction with some degreeof parallelism so thattheir areasof. contactwitheach other areelongated in the said direction. Similarly the adhesion between adjacent plies in a cylinder mold, multi-layered. web, tends to be greater because the fibers are alignedin the same general direction in the adjacent plies and hence have greater areas of contact with each other- By the same token when twowet webs in which the fibers have a different alignment are pressed together and dried, the adhesion between the plies will be lessened becauseof decreased areas of contact betweenadjacent, fibers.

It is possible for the strength of the bond between plies to be greater than. the weakest strength inside the plies. The weakest strength inside the ply is likely to be the strength of the fibers themselves, not the strength of the bond between fibers. The bond strength between plies can therefore, if high, exceed the fiber strength Within plies.

The area of contact between fibers either in a ply or as between plies is not the sole determinant. The amount of water in the web at the time of pressing the plies together is important in determining the area of contact between the fibers at the surface of the plies. In general the greater the amount of water'present during pressing, the greater the strength of the bond between fibers and plies, excepting that if too much wateris present. pressing may cause such movement between adjacent fibers as to be disruptive of, the. bond. Similarly, calendering a multi-ply web may reduce the bonding between the plies,

and'with a low initialbond strength the reduction upon calendering may be appreciable. Increasing the pressure on a wet multi-ply web tends to increase the bond strength, so long as the conidtions are such as not to produce movement or crushing. The rate of increase, however, is small above a, pressing force of 50 pounds per inch across the face of the press roll.

Assuming the bonding between fibers to be explainable on the theory of Campbell, it should follow that the degree of hydration has an effect thereon. By this is meant the degree to which water employed in forming the pulp enters into. combination. with. the cellulose of the fibers or tends partially to dissolve it. The degree of hydration is ordinarily dependent uponthe extent of the mechanical work exerted upon the fibers in the heating or refining operations to which the pulp is subjected. It is dependent, gtoo, upon the nature and composition of the fibers, some kinds hydrating readily, others not. A pulpwhich has been beatena minimum amount is known as a free stock. Water will drain from it comparatively readily. On the other hand a stock which has bee beaten actively a longtime is a highly hydrated stock. The surfaces of the fibers in it aresoftened or gelatinized,

and the stock is referred to as slow. When such. a

stock, is formed upon a screenthe water drains from it more slowly, and the removal of water is much more,

difficult.

Another action also occurs duringbeating. The me. chanical work involved fibrillates the fibers, which is to. say, splits and frays the ends and surfaces into .br.ushor branch-like fragments, largely remaining attached to the fibers, and thus exposes morecellulosic-surface tothe solvent action of water. Itis alsoknownthat a moderate content of noncellulose in the cellulosic fibrous material makes the pulp more easily beaten and increases thefibri-llating action. The slowness of a. slow pulp is probably due to the decreased rate of drainage of water from such a pulp due to the friction of the water against the.

increased exposed cellulosic surfaces. Additional slowness of drainage results if the fibers areshortenedwhen.

beaten.

It may. be pointed. out here that the couching pressure i.e. the pressure initially exerted when twofreshly made plies are brought into contact affects the bond between plies differently dependingupon the hydration. Witha highly hydrated stock the couching pressure appears slightly to decrease the bond, whereas with stocks of low hydration the bonding between plies increases as the couching pressure is increased.

The amount. of beating, i.e. the degree of hydration, is a very important factor in determining the amount of bonding beween fibers and between plies. All other conditions remaining the same, as the degree of beating is increased, the bonding between pliesalso increases. When the beating has been carried past a certain. point, however, the strength of the web decreases whereas the bonding between plies continues to increase. Atthis stage the slower or less free stock produces little orno change.

in the bonding if the freer stock remains unchanged. If the stock in a ply is extremely free, it will not bond. to a ply made of well beaten stock any better than it wil bond to another ply of very free stock.

When a suction drum mold layer. of felted fibers is placed against a layer of fibers formed on the conventional cylinder mold, it will be found that the two layers tend to adhere together in the finished anddried product to a measurably lesser degree than if bothlayers had been formed on cylinder molds. The reasons for. this are thought to be two-fold: first, fibersv formed. upon the suction drum mold have a differentorientation from those formed upon a cylinder mold. This may tend to decrease the contact area bet-ween the surface fibers of the adjacent plies. Second, the suction drum mold tends to close up a layer of fibers felted upon it more tightly than does a conventional cylinder mold. The decreased porosity of the suction drum mold layer, causes an increased water flow between this layer and the adjacent layer which, in the light of the considerations above, may tend to disrupt adhesion.

These factors, together with such. other factors as a difference in the nature of the stocks in the two plies and the degree of beating or hydration ofv each, tend to affect adhesion in various ways. Boards of low thickness, say, 0.020 in. andless, characterized by a suction,

These remedies are relatively of less etfectivenesswhen a suction drum mold layer is ioine d, toe, cylinder mold layer. As a paperboard designed for folding increases in thickness, the length of the fibers in the liners and particularly in the top liner must be preserved to maintain foldability, and this, in turn, makes it more diflicult to control adhesion, especially by control of the stock preparation. The skilled worker will understand that substantial increases in the degree of hydration by beating and refining generally have a marked tendency to decrease fiber length, although it is possible. to hydrate and preserve fiber length.

A sufi'icient degree of ply adhesion, particularly of the first and second liners of a folding paperboard, is very important, but is difficult to measure because it is complicated by considerations of the thickness, uniformity, and strength of the liner or ply itself. A paperboard can have good folding qualities even when top liner adhesion is poor if the liner is rich in long and strong fibers. It is a curious fact that lowered ply adhesion tends to promote folding quality regardless of the condition of the top liner stock. This is not to say that a top liner of poor quality per se is changed into a good folder by degrading ply adhesion. But the tensile strain placed on the top liner when a finished board is folded is actually somewhat relieved when the liner tends to detach from the adjoining ply when the board is folded. At the same time, a lack of adhesion of a liner may present difliculties in printing. It may be expected that the use of a tacky ink will tend to pick at a loose liner; but here again, additional considerations are involved. A tacky ink printed in a small area may not tend to disturb a loose liner while a softer ink printed over a very much larger area may tend to lift it. The importance of strong interply adhesion is also very great in the formation of glue joints in paperboard cartons. It may be pointed out that the nature of the liner itself exerts an apparent eifect for, if the liner is well formed and strong, it may be strippable from the layer beneath, giving an impression of poor adhesion, whereas a weaker top liner may tend to disintegrate or pull apart during stripping, giving perhaps an erroneous impression of greater adhesion.

A test has been developed by Stanley W. Trosset, Jr. of applicants assignee which is useful in measuring adhesion. A metal template with parallel sides, exactly one inch apart, is laid on the top liner of a piece of paperboard to be tested, extending in the machine direction; and a razor blade is used to cut cleanly down through the top and inner liners on each side of the template. The cuts are about 11 inches long and the board sample is usually 3% inches wide by 11 inches long. The sample of board treated in this fashion is fastened by broad metal clamps lengthwise on a wooden panel held in an inclined position at 30 degrees from the horizontal. The

easy peeling end of the board is at the lower end of V the panel. The peeling of the top liner is started upwardly, using the fingernails for a distance of an inch or so, whereupon the loose end of the liner is attached by pressure-sensitive tape to the base portion of a thin triangular piece of metal one inch wide and having a small eyelet or perforation at its apex. From the eyelet a string runs upwardly and backwardly over the panel and parallel thereto. The stripped portion of the liner is caused to pass around a roller /1 in. in diameter and made from a piece of hard rubber tubing which is wider than the peeled liner. Off the upper end of the panel the string passes around a small pulley, then vertically downwardly; and a light weight polyethylene container is attached-to its lower end.

To make the test, water is run into the container from a burette until the peeling of the liner proceeds slowly under the influence of the weight of the container and the water therein. The total weight of the container and the added water in grams is taken as the Trosset ply adhesion value per inch width of liner.

It is the nature of plies of paperboard to peel more easily from one direction than from the other lengthwise desired in the finished board. The

of the web. This is believed to be because the trailing ends of fibers in the machine direction tend to overlap each other like shingles, whereas their leading ends tend to be more firmly fastened. The operator can easily ascertain qualitatively by a hand test which direction is the easy peeling direction. When tested under the standard conditions described, a good bending paperboard should show a top liner adhesion of at least between and grams Trosset.

Where a deficiency in ply adhesion is encountered, despite such controls of the stock and machine operation as have been mentioned, it may be overcome by applying a suitable adhesive solution in water to the wet interface between the plies, using a revolving roll applicator for the adhesive solution. While other substances, inclusive of the known animal and vegetable adhesives, may be employed, starch, used as hereinafter taught, is the preferred adhesive of this invention, especially since its action is capable of exact control. A rotating, wire-screencovered metal applicator roll 13 is used to apply the starch solution to the top cylinder mold ply of the paper material 7 clinging to the under side of the moving bottom felt 8 between the cylinder mold 11 and the suction drum mold 1. A typical applicator roll extends across the web of the board machine, having a length, say, of 142 inches and a diameter of, say, 6 inches. It is supported on its ends by shafts in bearings and is driven so as to move with the board being made but at a slightly higher speed, say, 5 feet per minute faster. In an exemplary embodiment, the roll surface beneath the wire screening is cut longitudinally into 38 flutes, each 9 of are. wide with triangular walls sloping at 30 from the radius line, and having flat bottoms. The remaining degrees of arc (360642) form equal width, narrow surfaces on the tops of the walls to support a wire screen cloth of l4 meshes to the inch which cove-rs the outside of the roll.

The roll is suspended in a shallow metal trough 14 extending across the web. Starch solution from a supply tank (not shown) is pumped into the trough at one end; the rotating applicator roll, dipping into the solution for an inch or so, transfers it to the surface ply of the felted Web 7. The excess solution overflows from the other end of the trough. The excess may be treated as by a vibrating screen to remove fibers, dirt and the like, and returned to the supply tank.

In an exemplary embodiment, the starch solution was applied to the felted web 7 at about 6 feet from the last cylinder mold and about 7 feet from the succeeding suction drum mold. With the board machine operating at an exemplary speed of 275 lineal feet per minute, the adhesive solution thus was applied to the inner liner surface about 1.3 seconds after the inner liner was formed by the cylinder mold, and about 1.5 seconds before the top liner was applied by a suction press mold. In this exemplary embodiment, the dry solids content of the board web, to which the starch solution was applied, was about 11.5%, this figure being but little changed by the imposition of the starch solution. When the top liner ply was added, the solids content of the entire web was about 12%. These figures are not limiting, but are intended to be descriptive of the wet condition of the plies at the time the adhesive solution is applied, and when the plies are brought together. A figure of 13% solids is commonly accepted as usual for a felted ply delivered by a coventional cylinder mold.

In the application of the adhesive in accordance with p the teachings of this invention, control is had in various it can occur between the plies, and the degree of adhesion ways in view of the thickness of the paperboard, the nature of the plies, the natural adhesion to the extent that quantity of solution applied can be varied to suit requirements, but this may involve difliculty. It is preferable to apply the solution in fixed quantity but toflcoutml the adhesive action by varyand. as toits solids content. Thus, whil e the; coverage of the adhesive solution naturally will vary with the kind High r Q d on ents of starch solutions arensedion thicker boards according to this. guide, following:

and thickness of. the board, it will be found satisfactory j S rd C to maintain it in the general range of about to 1 7 gallons 5 Thickness Entire Board, Inches gg g ggfifi per thousand square feet of the ply surface to which it is tg Iercegt applied. With appropriate changes in the nature of the W tam solution, however, this range may be broadened. 55

The preparing of the starch adhesive is not difficult al-. fig 8 2%? j though it must be done carefully. and according to the re- .024 17 .0 1. 27:1: quirements of the total: thicknessof board being pasted. Ordinary pearl starch is preferred over the starches treated to .give lower viscosities in Water solution, and the pearl $3 33: ig g gz z g f fi ig gg ggisg gg gi 5 :55: Z grade tself gives better ply adhesion 1f 1t snot completely of modifying the Properties of the p of the board. ieued m sclutwn' A Suspension of pearl Stars? 15 For example a solution or dispersion of a substance actm W npt coqked g1 some ply 'adheslon but 0rd? ing to increase or decrease the. sizing or water resistance manly not. m satlsf-a-ctory amount Apparenjdy this of the top liner or to increase or decrease the absorption mun} pasting qondltlon uses pearl sitarch Such State and penetration of-water therein may be employed. A" of d-lspepslon m Water-ithat It Is d-eposlwd on the fibrous colorin substance or dye-stuff in solution or dispersion ply interfaces, to remain there dur ng the subsequent steps may bgapplied to color the top A clay or opacify of the.b0ard makmgiprocess Where It canpxert'maxlmum ing material like titanium dioxide rriay'be' used to mask adhesiveness, rather than belng forced. into the fibrous the ifiner liner and domribute to the brightness of the interfaces with consequent loss of adhesiveness. Moreish of the board However the use of apacifying over? the maxim emg incompletely Jeued as appiled to terials or other iinely divided solids must proceed with the interface 311mg proceedsas Paste? plies are caution since they have a distinct tendency to diminish heated. and dried. However th1s is. not to unply. that adhesion and in s me instances to Maw the liners apart. pastmg cannot-be done wlth other g h unless accompanied by the use of excessive quantities of convenient method of prepa-nngt e peayl sItal:c adhesive which is undesirable for various reasons. solution a concentrated master solut10n-1s made 1n which -y gy ofgexample wax Size 'added to the Starch the starchis cooked to 180 F.- (incompletely jelled), the I V 7 jelling is halted by adding cold Water, and quantitiesof 2 g gg surfacel 3 fi g 3;: as .22? make 1 e di ute starc so utions use to paste e 'ners. Y 7

For a typical master solutions 400 pounds of powdered creased the surfacebabsorption of \ivaterfron:i 2.06igranis pearl starch are slowly addedto SQO gallons of cold water F 2-836 C0 b 111 an P Y g 2 b we in a arge circu ar ta en t e mixture is agita e W1 the of reachesl8Q F. Imme iate y,t en, t e steamiss uto 1 3 and cold water (6040 F.) mixed into a total volume 40 P651011 from 0 7 g atf s Trolsget Wilil; the glycefln of 550 gallons. Theresult is a cooled, partially elled Increased P y adheslonblmm d h -f rth a out o weig t. L

For use 0; the board machine, a measured volume of Paperboards with great advantage, WhFTe the sumo the master starch solution is diluted with cold water to the} mold layer layers s? as herem taughtspecified solids content, in another large tank proby gl ggi 'o g iri g'rggi grg iggshgfy' aggpsgnfgui g gi 'd d th i'table stirrin device. 0 s contents Y 2 1$ vary o to A more formation, flatness thinness, smoothness of surface andy a roach to s uareness. A square sheet has strength complete description of these starchsolunons follows. cggracteristics EUChaSteHSHGJeaB sfifinesstetci the e Age l gi h d F h inthe direction of web travel as thesheetwas madeon Temperature used-in supply tank- 70-80 1 the paper machine: as in the directionperpendicular to. Temperature used-when applied to ply 80 Rt 7 T fijilrefi'tentned; themachine; (MD) and cross ma- Solids-conten-ts 0.55 to 1.2 0 c me I rec 1011s; Appearance l d A paper; ply made by the suction drumunold' may apa Color whi i h. preach squarenessmorethan one made by cylinder mold;, pH inSupp1ytank 15 and thereby more nearly resemble paper-made by the pHwhen applied'to ply 6.8: 'Fourdriniermachine in; that respect. However, it does not attain the squareness ofFourdrinier paper. For ex- The plus and minus signs indicate that practical variaample, note these typical (but not. exclusive) figures ofi tions from these figures will occur. which the ratios are significant:

Tensile Tear Density .Tmck- V I D658 2 MD. OD Ratio MD OD Ratio 12.1 as 3.1 1 2210;: 2353 .941 4.24 .00175 a tilt as 1 iii a 1 i cynndersmen 2612s; 5E3 510/1. 1' m f 8320 163 1. sfssi 10045 By rourdrmieranunm 29-6 15.4 1.9/1 106.,6 131.3 .81/1. 2.99 t .0050. Byscreen -.r 9.0 4.9 1:811 I 24.0 28.8 .8311 4.15- .0015

1 Pounds weight/M s11. 40.001 inch thickness. 1 Inches.

' Kra tnrappmen ner- Typewriter copy paper.

Formation, flatness and smoothness of paper plies, how ever made, are not easily specified numerically. Viewed by light transmitted through the web, a well-formed, continous, fiat, thin ply will show uniform distribution of fibers, absence of fiber clumps, no thick or thin spots as evidenced by absence of light and dark areas, and uniform opacity, whatever its degree. Smoothness is judged by appearance and feel, particularly of the dried ply. The ply-forming equipment permitting, the ply will have uniform thickness, and if both uniform and very thin, will have increased value. A good quality ply responds better to the pressing, smoothing, drying, and calendering op erations given it in the multi-ply board making operation.

Examined by experts, a ply made by suction drum mold is rated superior to that by cylinder screen mold in all the qualities just discussed. By calipering measurements of the dried ply, it is found to excel in flatness and evenness and to be producible unusually thin. Thinness of a ply, accompanied by other desirable properties, is valuable in the expensive top liner of paperboard which is made of costly high quality materials to provide good color, printing characteristics, strength, foldability, etc. It is found practicable to substitute a top liner ply with thickness in the range 0.0015 to 0.0025 inch when made by suction drum mold for one in range 0.0035 to 0.0055 inch and up made by cylinder screen mold and thus to produce a multi-ply paperboard of any conventional thickness, and with a superior top liner. Increased squareness of top liner ply is produced, too, by the suction drum mold. The result of all these improvements is a multi-ply paperboard with top ply of quality somewhat resembling that produced by the Fourdrinier and remaining plies of cylinder screen mold quality.

Indications are that the suction drum mold makes a ply similar to or slightly greater in density than the cylinder screen mold. As shown in the above table, densities of 4.24 and 3.78 pounds per M square feet per 0.001 inch thickness of ply for typical suction drum mold plies compare with 3.79 and 3.55 for typical cylinder screen plies.

The much higher suction on the restricted area of suction drum mold surface where the web is formed sets the fibers into place so rapidly that their opportunity to line up generally parallel to the machine direction is minimized. The result is a ply of paper with enhanced squareness.

Moreover, the construction of the suction drum mold with its high stability and strength and smooth, flat, level fine mesh wire screen surface provides a forming means which, continuously and evenly rotated, and assisted by appropriate mechanism to provide it with a constant supply of uniformly mixed, dilute fibrous stock, produces a corresponding high quality paper ply free of repetitive and sporadic defects at any speed of rotation of the suction drum mold. In contrast, with the conventional cylinder screen mold a speed of rotation is reached beyond which the formation of the paper ply becomes uneven, wild, non-uniform, etc., even to extents not correctible by changes in the paper pulp being used.

It is possible by expert operation of the cylinder screen mold to produce a very thin paper ply with average thickness similar to that made by a suction drum mold. However, the cylinder mold average thickness is derived from very thin as well as thick areas, and areas with poor and good formations having corresponding low and high opacities and abilities to cover the under liner of the board. Thus the average thicknes of very thin top liner plies made by cylinder screen mold is a misleading figure per se, and it must be revised sharply upwardly to be based upon a minimum thickness possessing sutficient opacity and for good formation to cover the under liner. In contrast, the top liner ply made by suction drum mold has excellent covering power and uniformly good formation at a very low and uniform thickness.

Although of outstanding advantage in the manufacture of folding grades of paperboard, this invention is useful 14, in making non-folding grades. Here the fibrous furnish of the board and the refining of it may detract from foldability, as is well known in the art, but the other advantages of a thin, fiat, even, uniform liner ply remain.

Modifications may be made in our invention without departing from the spirit thereof. Having described our invention in certain exemplary embodiments, what we claim as new and desire to secure by Letters Patent is:

1. In apparatus for producing a multi-ply paper product, the combination of a plurality of paper product producing machines comprising at least one rotary-cylinder mold assembly including a container for a pulp slurry, a cylinder continuously rotatable in said container for continuously receiving a deposit of uniformly oriented pulp slurry on the outer surface thereof, web means tangentially engaged on said cylinder for continuously receiving on one surface the deposit of uniformly oriented pulp slurry thereon and conveying it as a continuous damp web away from said cylinder, and a suction drum mold assembly comprising a hollow perforated drum roll rotatable about its longitudinal axis and tangentially engageable transversely of said one surface of said web means, a suction box extending axially of said cylindrical drum and fixed with respect to said drum roll, said suction box including a radially disposed suction chamber sealingly engaged at a sector portion at the inner surface of said cylindrical drum, said suction box including control means for providing a variable negative pressure in said suction chamber through said perforated drum roll, said sector portion defined by said suction chamber being located immediately adjacent and forwardly of the line of tangential engagement between said one surface of said web means and said drum roll, and a pulp slurry feed conduit for distributing dilute, randomly oriented pulp stock on the outer surface of said drum roll in overlying relation to the sector portion controlled by said suction chamber for continuously producing a layer of paper product material on the outer surface of said drum roll and in which the moisture content thereof is controlled by said variable negative pressure in said suction chamber, said pulp slurry conduit including a discharge opening extending substantially the length of said sector portion at the outer surface of said perforated roll and including a lower bordering-lip parallel to and substantially sealingly engaging a lower bordering edge of said sector portion, said discharge opening including an upper bordering edge parallel to and substantially spaced from the upper border of said sector portion to permit an excess of said randomly-oriented pulp slurry to pass therebetween whereby the continuously formed damp web of uniformly oriented pulp on said web means is adhesively united with the continuously formed deposit of randomly oriented pulp formed on said suction roll.

2. In apparatus set forth in claim 1; liquid adhesiveapplying means disposed intermediately of said web means between said cylinder mold assembly and said suction drum mold assembly and including a portion for continuously applying a coating of a supplement adhesive to the continuous web of uniformly oriented damp pulp on said one surface of said web means.

3. In apparatus as set forth in claim 1; and pressure roll means disposed in overlying tangential relationship to said cylinder and drum roll and at the web means opposite said one side of said web means.

References Cited in the file of this patent UNITED STATES PATENTS 242,253 Benton May 31, 1881 1,621,888 Kirschbraun Mar. 22, 1927 1,682,826 Bidwell Sept. 4, 1928 1,718,574 Millspaugh June 25, 1929 1,799,350 Barnes Apr. 7, 1931 1,863,707 Wood June 21, 1932 (Other references on following page) 15 UNITED STATES PATENTS Pattillo et a1. Feb. 21, 1933 Osborne Jan. 28, 1947 Caesar et a1. Feb. 7, 1950 Wenzelberger et a1. Mar. 2, 1954 Perry Feb. 22, 1955 OTHER REFERENCES Witham Modern Pulp and Paper Making, Second edition (1942), pages 66, 340 and 541, published by Rein hold Publishing Co., 330 W. 42nd Street, New York, N.Y. 

